The subject matter disclosed herein relates to constant speed drives. More specifically, the subject matter disclosed herein relates to improvements in the operating range of constant speed drives.
In a typical constant speed drive (CSD), input power is taken from a prime mover (such as an aircraft engine) which operates at a variable speed. The speed of the prime mover can be referred to as a base shaft speed. Power from the prime mover (i.e., base shaft) is fed through a speed-sum differential resulting in output power at a constant output speed over a range of the base shaft speed for which the differential is sized. This output power is often used to drive a component, such as an electrical generator, which operates at constant speed.
Because the base shaft is driven over a range of speeds, a speed variator is used in conjunction with the differential. The variator is connected to the differential via a trim shaft, which is driven at a proportion of the base shaft speed determined by the variator. In some systems, the variator is a hydraulic unit (pump-motor set). For example, variators commonly allow for a mapping of output speed to a 2-to-1 input speed range, which is useful for power extraction from the high pressure spool of the engine. Other applications, however, require greater operating ranges of the CSD. For example, constant frequency (CF) generators extracting power from the low pressure spool of the engine require mapping of output speed to about a 5-to-1 input speed range, while engine electric start systems using the CF electric machine as a synchronous motor require mapping the constant motor speed to the entire speed range of the engine high pressure spool from zero to maximum speed. Increasing the input speed range of a CSD, however, typically has an over-proportional effect on size and weight of the speed variator.